I did it without lieing to the demon, though it took me a couple tries. The first time through I saved right before I entered the room (Hmm, magical barrier on the door, maybe I should save), talked to the demon, tried to lie to it so it would let the girl go but it didn't believe me so it possessed the kid and I killed it. I reloaded and agreed to let it out if it let the kid go, pissed off Alistair but I solved the puzzle then it possessed the girl anyway and left the room. Third try I didn't promise up front to release it but solved the puzzle, when the demon tried to change it's mind I told the girl to get out, she freaks out and leaves, the demon manifests and I toast it, all is right with the world. If I had better Persusion or higher Cunning my first try might have worked but I was able to get the job done anyway.